Carbonization of poly(arylene sulfide) electrical insulation materials can occur under the influence of high electrical potential. In application the surface of an insulator typically accumulates dusts, moisture, oils and so on. This accumulation of contaminants can create a conductive layer on the outer surface of the insulation material which, under the influence of sufficient potential drop, will "breakdown electrically" thus allowing the passage of current through the conductive layer and along the insulation surface. Heat, generated by the resistive dissipation of electrical energy as current passes through the conductive layer, causes decomposition or evaporation of the conductive medium. The passage of current, which may last for only a very short period of time, often exists in the form of an electrical arc across the surface of the insulator. The resultant high temperature along the current path promotes carbonization of carbonizable insulation material. This carbonization produces a carbon path which, if allowed to develop under the influence of successive surface tracking breakdown cycles, will ultimately produce a continuously conductive path which in turn will allow the continuous passage or leakage of current along the surface of poly(arylene sulfide) insulation material.
The resistance of an electrical insulation material to carbonization and the formation of conductive tracks can be measured by, for example, a Comparative Tracking Index test. The ASTM Test Method designated D 3638-77 and entitled "Comparative Tracking Index of Electrical Insulating Materials" describes the test in detail and is incorporated by reference herein. A description of the test can be found in the 1982 Annual Book of ASTM Standards, Part 39. The test is summarized therein as follows:
The surface of a specimen of electrical insulating material is subjected to a low-voltage alternating stress combined with a low which is dropped between two opposing electrodes every 30 s. The voltage applied across these electrodes is maintained until the current flow between them exceeds a predetermined value which constitutes failure. Additional specimens are tested at other voltages so that a relationship between applied voltage and number of drops to failure can be established through graphical means. The numerical value of the voltage which causes failure with the application of 50 drops of the electrolyte is arbitrarily called the comparative tracking index. This value provides an indication of the relative track resistance of the material. PA0 Electrical equipment may fail as a result of electrical tracking of insulating material that is exposed to various contaminating environments and surface conditions . . . . This method is an accelerated test which at relatively low test voltages, provides a comparison of the performance of insulating materials under wet and contaminated conditions . . . . PA0 When organic electrical insulating materials are subjected to conduction currents between electrodes on their surfaces, many minute tree-like carbonaceous paths or tracks are developed near the electrodes. These tracks are oriented randomly, but generally propagate between the electrodes under the influence of the applied potential difference. Eventually a series of tracks spans the electrode gap, and failure occurs by shorting of the electrodes.
The significance of the ASTM test is described, in part, as follows: